The present invention relates to an electrically writable nonvolatile semiconductor memory device, particularly one capable of setting the threshold potential of a cell transistor assigned to a writing operation rapidly and accurately.
A flash memory is attracting increasing attention as a memory for allowing data to be rewritten and for holding data even at the time of power down. To write data in a flash memory, either a channel hot electron injection scheme or a Fowler-Nodeheim tunneling scheme is often used. These two schemes both set the threshold voltage of a cell transistor corresponding to a write address at a preselected potential, and determine data to be read on the basis of a current to flow through the transistor and derived from the above potential.
There has recently been proposed a multivalue semiconductor memory device capable of holding a number of bit data in a single cell transistor due to enhanced integration. Data is written to a multivalue semiconductor memory device in basically the same manner it is written to a flash memory. The difference is that the multivalue semiconductor memory device must have its threshold voltage set with accuracy, and therefore must repeat the write confirming operation more frequently than the flash memory.
However, with any one of the conventional electrically writable nonvolatile semiconductor memory devices, it is difficult to minimize the substantial period of time necessary for the writing operation to be completed, and to set a desired threshold with accuracy.